Expose wait4 as wait4 rather than __wait4.

This helps strace(1) compile with one fewer hack.

Change-Id: I5296d0cfec5546709cda990abd705ad33d7c4626
diff --git a/libc/arch-arm/syscalls/__wait4.S b/libc/arch-arm/syscalls/wait4.S
similarity index 91%
rename from libc/arch-arm/syscalls/__wait4.S
rename to libc/arch-arm/syscalls/wait4.S
index ac59045..7011099 100644
--- a/libc/arch-arm/syscalls/__wait4.S
+++ b/libc/arch-arm/syscalls/wait4.S
@@ -3,7 +3,7 @@
 #include <machine/asm.h>
 #include <sys/linux-syscalls.h>
 
-ENTRY(__wait4)
+ENTRY(wait4)
     mov     ip, r7
     ldr     r7, =__NR_wait4
     swi     #0
@@ -12,4 +12,4 @@
     bxls    lr
     neg     r0, r0
     b       __set_errno
-END(__wait4)
+END(wait4)